Method and system for focus adjustment of a multi-beam scanning electron microscopy system

ABSTRACT

A scanning electron microscopy system is disclosed. The system includes a multi-beam scanning electron microscopy (SEM) sub-system. The SEM sub-system includes a multi-beam electron source configured to form a plurality of electron beams, a sample stage configured to secure a sample, an electron-optical assembly to direct the electron beams onto a portion of the sample, and a detector assembly configured to simultaneously acquire multiple images of the surface of the sample. The system includes a controller configured to receive the images from the detector assembly, identify a best focus image of images by analyzing one or more image quality parameters of the images, and direct the multi-lens array to adjust a focus of one or more electron beams based on a focus of an electron beam corresponding with the identified best focus image.

CROSS-REFERENCE TO RELATED APPLICATION

The present application is related to and claims benefit of the earliestavailable effective filing date from the following applications: Thepresent application constitutes a divisional patent application of U.S.patent application Ser. No. 15/272,194, filed Sep. 21, 2016, entitledMETHOD AND SYSTEM FOR FOCUS ADJUSTMENT A MULTI-BEAM SCANNING ELECTRONMICROSCOPY SYSTEM, naming Doug K. Masnaghetti, Richard R. Simmons, ScottA. Young, Mark A. McCord, and Rainier Knippelmeyer as inventors, whichis incorporated herein by reference in the entirety and is a regular(non-provisional) patent application of U.S. Provisional ApplicationSer. No. 62/222,325, filed Sep. 23, 2015, entitled TECHNIQUES FOR RAPIDFOCUS ADJUSTMENT IN A MULTIPLE-BEAM IMAGING SYSTEM, naming Mark McCord,Rainer Knippelmeyer, Douglas Masnaghetti, Richard Simmons, and ScottYoung as inventors, which is incorporated herein by reference in theentirety.

TECHNICAL FIELD

The present invention generally relates to scanning electron microscopy,and, in particular, to focus adjustment in a multi-beam electronmicroscopy system.

BACKGROUND

The fabrication of semiconductor devices, such as logic and memorydevices, typically includes processing a substrate such as asemiconductor wafer using a large number of semiconductor fabricationprocesses to form various features and multiple levels of thesemiconductor devices. As semiconductor device size become smaller andsmaller, it becomes critical to develop enhanced inspection and reviewdevices and procedures. One such inspection technology includes electronbeam based inspection systems, such as, scanning electron microscopy(SEM). In a single beam SEM, performing focus adjustments involvestaking multiple images at different focus settings, and then choosingthe best image (or interpolating between images to find the best focus).Frequently, astigmatism correction is also adjusted, which can furtherincrease the number of images required. The time to acquire images canbe relatively long, which reduces availability of the inspection tool.It would be advantageous to provide a multi-beam SEM system and methodthat cures the shortcomings observed in single beam SEM approaches.

SUMMARY

A multi-beam scanning electron microscopy apparatus with focusingadjustment capabilities is disclosed, in accordance with one or moreembodiments of the present disclosure. In one embodiment, the systemincludes a multi-beam scanning electron microscopy sub-systemcomprising: a multi-beam electron source configured to form a pluralityof electron beams; a sample stage configured to secure a sample; anelectron-optical assembly including a set of electron-optical elementsconfigured to direct at least a portion of the plurality of electronbeams onto a portion of the sample; and a detector assembly configuredto simultaneously acquire a plurality of images of the surface of thesample, each image associated with an electron beam of the plurality ofelectron beams. In another embodiment, the system 100 includes acontroller including one or more processors configured to execute a setof program instructions stored in memory. In another embodiment, the oneor more processors are configured to for causing the one or moreprocessors to: receive the plurality of images from the detectorassembly; identify at least one of a best focus image or a bestastigmatism image of the plurality of images by analyzing one or moreimage quality parameters of at least some of the images of the pluralityof the images; and direct the multi-beam source to adjust at least oneof focus or astigmatism of one or more electron beams based on at leastone of focus or astigmatism of an electron beam corresponding with atleast one of the identified best focus image or the identified bestastigmatism image.

In another embodiment, the controller directs the multi-lens arrayassembly to establish a focus gradient across an image field of thesample, wherein two or more lenses of the multi-lens array focus two ormore electron beams of the plurality of electron beams to differentfoci.

In another embodiment, the controller directs the multi-lens arrayassembly to establish an astigmatism gradient across an image field ofthe sample, wherein two or more lenses of the multi-lens array focus twoor more electron beams of the plurality of electron beams so as todisplay different amounts of astigmatism.

A multi-beam scanning electron microscopy apparatus with focusingadjustment capabilities is disclosed, in accordance with one or moreembodiments of the present disclosure. In one embodiment, the systemincludes a multi-beam scanning electron microscopy sub-systemcomprising: a plurality of multi-beam electron sources configured toform a plurality of electron beams; a sample stage configured to securea sample; an electron-optical assembly including a set ofelectron-optical elements configured to direct at least a portion of theplurality of electron beams onto a portion of the sample; and a detectorassembly configured to simultaneously acquire a plurality of images ofthe surface of the sample, each image associated with an electron beamof the plurality of electron beams. In another embodiment, the systemincludes a controller including one or more processors configured toexecute a set of program instructions stored in memory for causing theone or more processors to: receive the plurality of images from thedetector assembly; identify at least one of a best focus image or a bestastigmatism image of the plurality of images by analyzing one or moreimage quality parameters of at least some of the images of the pluralityof the images; and direct one or more electron-optical elements toadjust at least one of focus or astigmatism of one or more electronbeams based on at least one of focus or astigmatism of an electron beamcorresponding with at least one of the identified best focus image orthe identified best astigmatism image.

A multi-beam scanning electron microscopy apparatus with focusingadjustment capabilities is disclosed, in accordance with one or moreembodiments of the present disclosure. In one embodiment, the systemincludes a multi-beam scanning electron microscopy sub-systemcomprising: a multi-beam electron beam source including an electron gunconfigured to generate an illumination beam and a multi-lens arrayassembly configured to split the illumination beam into a plurality ofelectron beams, wherein the multi-lens array assembly is configured foradjusting focus of one or more lenses of the lens array assembly; asample stage configured to secure a sample; an electron-optical assemblya set of electron-optical elements configured to direct at least aportion of the plurality of electron beams onto a portion of the sample;and a detector assembly configured to simultaneously acquire a pluralityof images of the surface of the sample, each image associated with anelectron beam of the plurality of electron beams. In another embodiment,the system includes a controller including one or more processorsconfigured to execute a set of program instructions stored in memory forcausing the one or more processors to: direct the multi-lens arrayassembly to sweep at least one of the focus or astigmatism of one ormore lenses during acquisition of one or more images corresponding withthe one or more lenses; receive the one or more images from the detectorassembly; identify a point in the one or more images displaying at leastone of best focus or best astigmatism by analyzing one or more imagequality parameters across the one or more images; and direct themulti-lens array to adjust at least one of focus of one or more electronbeams or astigmatism of the one or more electron beams based on at leastone of the focus or astigmatism at the point in the one or more imagesdisplaying at least one of the identified best focus or the identifiedbest astigmatism.

A multi-beam scanning electron microscopy apparatus for measuring andcompensating for drift is disclosed, in accordance with one or moreembodiments of the present disclosure. In one embodiment, the systemincludes a multi-beam scanning electron microscopy sub-systemcomprising: a multi-beam electron beam source including an electron gunconfigured to generate an illumination beam and a multi-lens arrayassembly configured to split the illumination beam into a plurality ofelectron beams, wherein the multi-lens array assembly is configured foradjusting focus of one or more lenses of the lens array assembly; asample stage configured to secure a sample; an electron-optical assemblya set of electron-optical elements configured to direct at least aportion of the plurality of electron beams onto a portion of the sample;and a detector assembly configured to simultaneously acquire a pluralityof images of the surface of the sample, each image associated with anelectron beam of the plurality of electron beams. In another embodiment,a controller including one or more processors configured to execute aset of program instructions stored in memory for causing the one or moreprocessors to: direct the multi-beam scanning electron microscopysub-system to acquire a first image in an under-focused condition and anadditional image in an over-focused condition; receive the first imageacquired in the under-focused condition and the additional imageacquired in the over-focused condition from the detector assembly;identify focus drift in a current image by comparing the first image andthe additional image to the current image acquired at the current focus;and direct the multi-lens array to adjust a focus of one or moreelectron beams to compensate for the identified focus drift.

A multi-beam scanning electron microscopy apparatus for focus adjustmentis disclosed, in accordance with one or more embodiments of the presentdisclosure. In one embodiment, the system includes a multi-beammicroscopy sub-system to perform a line scan of a plurality of electronbeams comprising: a multi-beam electron source configured to form theplurality of electron beams; a sample stage configured to secure asample; an electron-optical assembly including a set of electron-opticalelements configured to direct at least a portion of the plurality ofelectron beams onto a portion of the sample; and a detector assemblyconfigured to simultaneously acquire a plurality of images of thesurface of the sample, each image associated with an electron beam ofthe plurality of electron beams. In another embodiment, the systemincludes a controller including one or more processors configured toexecute a set of program instructions stored in memory for causing theone or more processors to: receive a plurality of line scans from thedetector assembly; identify at least one of a best focus line scan or abest astigmatism line scan of the plurality of line scans by analyzingone or more line scan parameters of at least some of the line scans ofthe plurality of line scans; and direct the multi-lens source to adjustat least one of focus or astigmatism of one or more electron beams basedon at least one of focus or astigmatism of an electron beamcorresponding with at least one of the identified best focus line scanor the identified best astigmatism line scan.

BRIEF DESCRIPTION OF THE DRAWINGS

The numerous advantages of the disclosure may be better understood bythose skilled in the art by reference to the accompanying figures inwhich:

FIG. 1A is a block diagram view of a multi-beam scanning electronmicroscopy system with focus and astigmatism adjustment capabilities, inaccordance with one embodiment of the present disclosure.

FIG. 1B is a block diagram view of a multi-beam scanning electronmicroscopy system with focus and astigmatism adjustment capabilities, inaccordance with one embodiment of the present disclosure.

FIG. 2 is conceptual view of a set of SEM images having different focus,in accordance with one embodiment of the present disclosure.

FIG. 3 is conceptual view of a set of SEM images with one or more imagesdisregarded due to the lack of structural features, in accordance withone embodiment of the present disclosure.

FIGS. 4A-4D are simplified schematic illustrations of the application ofa focus gradient across at least one dimension of a sample, inaccordance with one embodiment of the present disclosure.

FIG. 5 is conceptual view of an SEM image that was swept in focus duringimage acquisition, in accordance with one embodiment of the presentdisclosure.

FIG. 6 is conceptual view of a set of SEM images used to analyze focusdrift in a multi-beam SEM system, in accordance with one embodiment ofthe present disclosure.

FIG. 7 is conceptual view of a set electron beam lines scans used forrapid focus adjustment of a multi-beam electron beam system, inaccordance with one embodiment of the present disclosure.

FIG. 8 is a process flow diagram illustrating a method for rapid focusadjustment in a multi-beam SEM system, in accordance with one or moreembodiments of the present disclosure.

FIG. 9 is a process flow diagram illustrating a method for rapid focusadjustment in a multi-beam SEM system, in accordance with one or moreembodiments of the present disclosure.

FIG. 10 is a process flow diagram illustrating a method for rapid focusadjustment in a multi-beam SEM system, in accordance with one or moreembodiments of the present disclosure.

FIG. 11 is a process flow diagram illustrating a method for rapid focusadjustment in a multi-beam SEM system, in accordance with one or moreembodiments of the present disclosure.

FIG. 12 is a process flow diagram illustrating a method for rapid focusadjustment in a multi-beam SEM system using a set of electron beam linescans, in accordance with one or more embodiments of the presentdisclosure.

DETAILED DESCRIPTION OF THE INVENTION

Reference will now be made in detail to the subject matter disclosed,which is illustrated in the accompanying drawings. Referring generallyto FIGS. 1 through 12, a system and method for rapid focus adjustmentsin a multi-beam scanning electron microscopy (SEM) imaging system isdescribed in accordance with the present disclosure.

Embodiments of the present disclosure are directed to providingautomatic methods for quickly focusing a multi-beam SEM system. In amulti-beam SEM system, a large number of sub-images (e.g., 2 to 200) maybe acquired simultaneously that together form a larger contiguous image.In some embodiments of the present disclosure, multiple sub-imageshaving varying focus and/or astigmatism characteristics are acquired.Based on these images, embodiments of the present disclosure identifyoptical settings for achieving the best focus and/or least amount ofastigmatism in images acquired with the multi-beam SEM system of thepresent disclosure.

FIG. 1A illustrates a system 100 for performing multi-beam SEM imaging,in accordance with one or more embodiments of the present disclosure. Inone embodiment, the system 100 includes a multi-beam scanning electronmicroscopy (SEM) sub-system 101 and a controller 110. The multi-beam SEMsub-system 101 may include any multi-beam SEM sub-system or multi-beamSEM tool known in the art. For example, the multi-beam SEM sub-system101 may include, but is not limited to, a multi-beam electron beamsource 103, an electron-optical assembly 104, a sample stage 106, and adetector assembly 108. In another embodiment, the controller 110 iscommunicatively coupled to the multi-beam SEM sub-system 101. Forexample, the controller 110 may be coupled to the output of the detectorassembly 108 of the SEM sub-system 101.

In one embodiment, the multi-beam electron source includes an electrongun 102 and a multi-lens array assembly 109 configured to split aninitial illumination beam into multiple electron beams 105. For example,the electron gun 102 may include, but is not limited to, a fieldemission gun (cathode). By way of another example, as shown in FIG. 1A,the multi-lens array assembly 109 includes a set of lenses 111 (or“lenslets”) in an array plate 113. In this regard, the set of lenses 111serve to split the initial illumination electron beam into multiplebeams 105 (or “beamlets”).

In another embodiment, the multi-beam array assembly 109 is adjustable.For example, the multi-beam array assembly 109 is configured toindividually adjust and/or control the focus and/or astigmatism of oneor more lenses 111 of the lens array assembly 109. In this embodiment,the individual lenses 111 of the set of lenses of the array assembly 109may focus each beam independently.

In another embodiment, the detector assembly 108 simultaneously acquiresmultiple images (or “sub-images”). In this regard, each of the electronbeams 105 causes a corresponding electron signal (e.g., secondaryelectron signal or backscattered electron signal) to form a set ofsignal beams 117. The signal beams 117 then form a set of correspondingimages, or sub-images, at the detector assembly 108. The images acquiredby the detector assembly 108 are then transmitted to the controller 110for focus and/or astigmatism analysis. For example, in the case wherethe SEM sub-system 101 includes N simultaneously operating beams (e.g.,2 to 200 beams), where N corresponding images are simultaneouslyacquired by the detector assembly 108, the N images can be analyzed toanalyze focus and/or astigmatism of one or more of the N images.

The controller 110 may be coupled to the output of the detector assembly108 in any suitable manner (e.g., by one or more transmission mediaindicated by the line shown in FIG. 1) such that the controller 110 canreceive the output acquired by the detector assembly 108. In oneembodiment, the controller 110 includes one or more processors 116 and amemory medium 118 (or memory). The one or more processors 116 areconfigured to execute a set of program instructions maintained in thememory medium 118 for causing the one or more processors to carry outone or more of the various steps described through the presentdisclosure.

In one embodiment, the program instructions are configured to cause theone or more processors to utilize information from two or more imagesacquired by the detector assembly 108 to identify the best focus of SEMsub-system 101. In one embodiment, the program instructions areconfigured to cause the one or more processors to receive multiple SEMimages from the detector assembly 108. It is noted that two or moreimages of the set of images (e.g., Image1-ImageN) have a different focuscondition. As a result, images with different focus conditions (e.g., infocus, over-focused, or under-focused) will display a different level ofimage sharpness. In another embodiment, the program instructions areconfigured to cause the one or more processors to identify a best focusimage of the set of images by analyzing one or more image qualityparameters of at least some of the images (e.g., all of the images or asub-set of the images) received from the detector assembly 108. Forexample, the one or more processors may identify a best focus image ofthe Images1-ImagesN by determining image sharpness for each of theImages 1-Images N received by the controller 110. Then, the one or moreprocessors 116 may identify the image(s) displaying the best imagesharpness as the image having the so called “best focus.” It is notedthat any image analysis technique known in the art to determine imagesharpness may be used by the various embodiments of the presentdisclosure. In another embodiment, once a best focus image isidentified, the controller 110 may then adjust one or more lenses 111 ofthe multi-lens array 113 so to match the focus settings of the lens(es)used to form the best focus image.

It is noted that, while the present disclosure focuses on a SEMsub-system 101 that generates multiple electron beams using an electrongun and multi-lens array, this configuration should not be interpretedas a limitation on the scope of the present disclosure. It is recognizedherein that multi-beam sub-system 101 may generate multiple electronbeams in any manner known in the art.

For example, as shown in FIG. 1B, the multi-beam source 103 may includea set of individual electron sources 102 a-102 d (e.g., set of electronguns) to form an electron source array. Further, although not shown,each channel of the system 100 may include an extraction electrode and alens electrode to form each of the multiple beams 105. In addition, oneor more electron-optical elements of the electron-optical assembly 104may serve to adjust/control the focus of one or more of the electronbeams 105. For instance, a condenser lens 124 of the electron-opticalassembly 104 may serve to adjust/control the focus of one or more of theelectron beams 105 in response to controller 110. For purposes ofsimplicity, the remainder of the present disclosure focuses on thegeneration of multiple electron beams using the multi-lens source ofFIG. 1A, however, it is noted that any of the embodiments describedthroughout the present disclosure may be extended to the multi-beamsource of FIG. 1B and variations thereof.

FIG. 2 illustrates a conceptual view of a series of images 202 a-202 dacquired simultaneously with the multi-beam SEM sub-system 101, inaccordance with one or more embodiments of the present disclosure. Asshown in FIG. 2, focus varies across the simultaneously acquired images202 a, 202 b, 202 c and 202 d. In the example depicted in FIG. 2, image202 b displays the best image sharpness and, thus, the best focus. Afterthe controller identifies image 202 b as the best focus image, thecontroller 110 may then adjust the focus settings of the lenses 111 ofthe multi-beam lens array 113 so as to match the focus setting of thelens used to form image 202 b. It is noted that the focus variation maybe imparted to the field and, thus, the image naturally orintentionally, as discussed in more detail further herein.

It is noted that in some instances one or more images acquired by thedetector assembly 108 lack sufficient structural features to analyzeimage sharpness in the particular image. In one embodiment, prior toidentifying best focus of the set of images from the detector assembly108, the controller 110 may ignore one or more images that lacksufficient structural features for determining focus of the one or moreimages. In such cases, images containing insufficient structuralfeatures for proper image sharpness analysis, these images may bedisregarded or ignored for the purposes of identifying the best focusimage.

In one embodiment, the program instructions are configured to cause theone or more processors to ignore or disregard one or more imagesacquired by detector assembly 108. FIG. 3 illustrates a conceptual viewof a series of sets images 302 a-302 d acquired simultaneously with themulti-beam SEM sub-system 101, in accordance with one or moreembodiments of the present disclosure. It is noted that the image 302 cdepicts a portion of the sample 107 that lacks sufficient structuraldetail to analyze image sharpness/focus quality of the image. In oneembodiment, images that lack sufficient structural detail for imagesharpness/focus quality may be disregarded or ignored by the controller110. In this regard, the controller 110 may carry out the best focusdetermination described above without images, such as image 302 c, thatare blank or contain insufficient structural features.

It is noted that focusing a single beam SEM often requires moving tospecific focus targets because a single image may not contain enoughfeatures to allow reliable focusing. However, in the multi-beam SEMsystem 100 of the present disclosure, the controller 110 may ignoresub-images that are blank or lack sufficient image features. It isfurther noted that due to the large number of sub-images, it is unlikelythat all or even most of the sub-images would lack some features forfocus. As a result, in many cases, separate focus targets would becomeunnecessary.

It is noted that, while the embodiments described above have focused onthe identification of the best focus condition for the multi-beam SEMsub-system 101, this should not be interpreted as a limitation on thescope of the present disclosure. For example, analogous techniques maybe applied by the various embodiments of the present disclosure todetermine the lens configuration to achieve minimal astigmatism.

It is noted herein that the focus and/or astigmatism variation acrossthe set of sub-images acquired with the detector assembly 108 may beestablished through any suitable mechanism.

In one embodiment, the program instructions are configured to cause theone or more processors to utilize natural occurring focus and/orastigmatism variation in determining the best focus of SEM sub-system101. In some embodiments, the focus variation across the sample 107 (andthus the images acquired by the detectors 108) may be established bynaturally occurring focus variation. For example, naturally occurringfocus variation may be caused by field curvature aberration. Forinstance, field curvature aberration may impart focus variation amongthe sub-images acquired by detector assembly 108. In this example, thecontroller 110 may analyze the sub-images Image1-ImageN so as toidentify the image with the best focus and, thus, the focus conditionleading to the best focus without intentionally introducing a focusgradient.

It is noted that such image variation would need to be small enough tonot noticeably degrade the performance of the SEM system 100. In a givenimage field, it is further noted that field curvature typically resultsin the outer sub-images being over-focused, while the center sub-imagesare under-focused. In this case, in some embodiments, the best overallfocus may be obtained by selecting a focus that is intermediate betweenthe best focus for inner sub-images and the best focus for outersub-images. This technique has the additional advantage of being able tobe performed on actual user images, not necessary requiring separatespecial images for identifying best focus.

It is further noted that system 100 may take advantage of naturallyoccurring astigmatism. For example, naturally occurring astigmatismvariation may be caused by off-axis astigmatism aberration. Forinstance, off-axis astigmatism aberration may impart astigmatismvariation among the sub-images acquired by detector assembly 108. Inthis example, the controller 110 may analyze the sub-imagesImage1-ImageN so as to identify the image with the least amount ofastigmatism and, thus, the lens condition leading to the least amount ofastigmatism.

It is noted that for the purposes of the present disclosure the term“naturally occurring focus variation” is interpreted to mean variationin focus that occurs without intentional variation in focus caused bythe system 100 or a user. Similarly, the term “naturally occurring focusvariation” is interpreted to mean variation in astigmatism that occurswithout intentional variation in astigmatism caused by the system 100 ora user.

Referring again to FIG. 1A, in another embodiment, the programinstructions are configured to cause the one or more processors toutilize intentionally occurring focus and/or astigmatism variation indetermining the best focus of SEM sub-system 101.

In one embodiment, the multi-lens array assembly 109 may introduce afocus and/or astigmatism gradient across the image field of the sample107. In this regard, the controller 110 may direct the multi-lens arrayassembly to establish a focus and/or astigmatism gradient across animage field of the sample 107.

In another embodiment, after establishing a focus and/or astigmatismgradient across the image field of the sample, a large number ofsub-images (e.g., 2 to 200 sub-images) may be acquired simultaneously,whereby each (or at least some) of the sub-images are acquired by thedetector assembly 108 at different foci and/or different astigmatism. Inturn, each of the acquired sub-images may be analyzed by controller 110for image sharpness (or other focus indicator) to find the best focussetting and/or best astigmatism setting.

In another embodiment, once a best focus and/or astigmatism setting isidentified, the controller 110 may then direct the multi-lens arrayassembly 109 to adjust a focus and/or astigmatism of one or moreelectron beams 105 based on the focus of an electron beam correspondingwith the identified best focus image.

FIGS. 4A-4D illustrate a variety of configurations for inducing a focusgradient across the image field of the sample 107, in accordance withone or more embodiments of the present disclosure. FIG. 4A depicts anominal configuration 400, whereby the multi-lens array assembly 109does not impart a focus and/or astigmatism gradient to the electronbeams 105.

In one embodiment, as shown in view 410 of FIG. 4B, the multi-lensassembly 109 may vary the focus across an image field by varying thevoltages across the set of lenses 111 of the multi-lens assembly 109 inone or more directions. It is noted that by varying the voltages of thevarious lenses 111 across the multi-lens assembly 109 the strength ofeach lens may be varied in a corresponding fashion. In this regard, afocus gradient may be established across the multi-lens assembly 109, asshown in FIG. 4B.

In another embodiment, as shown in view 420 of FIG. 4C, the multi-lensassembly 109 may vary the focus across an image field by mechanicallyvarying the gaps in the plates of the lenses 111 across the multi-lensarray 109. It is noted that by varying the plate gap in the variouslenses 111 across the multi-lens assembly 109 the strength of each lensmay be varied in a corresponding fashion. In this regard, a focusgradient may be established across the multi-lens assembly 109, as shownin FIG. 4C.

In another embodiment, as shown in view 420 of FIG. 4D, the multi-lensassembly 109 may vary the focus across an image field by mechanicallytilting the multi-lens array 109. It is noted that by varying tilt ofthe multi-lens assembly 109 the focus of each beam 105 may be varied ina corresponding fashion. In this regard, a focus gradient may beestablished across the multi-lens assembly 109, as shown in FIG. 4D.

In one embodiment, the change in voltage, lens gap, and tilt istemporary. In this regard, the changes may be imparted temporarily inorder for the detector assembly 108 to acquire multiple SEM images atdifferent focus and transmit the image results to controller 110. Then,once the controller 110 has identified the best focus condition, thecontroller 110 may then direct the multi-lens assembly 109 to adjust allor at least some of the lenses to match the focus setting of the bestfocus image.

While for purposes of simplicity the imparted focus variations have beenshown only in one dimension in FIGS. 4B-4D, it is noted herein that sucha configuration is not a limitation on the scope of the presentdisclosure. For example, focus may vary as a function of lens positionin two dimensions (e.g., X- and Y-positions).

It is further noted that while FIGS. 4A-4D have been described in thecontext of varying focus across the multi-lens assembly 109, itrecognized herein that system 100 may establish an astigmatism gradientacross the image field such that each sub-image has a slightly differentamount of astigmatism.

Referring again to FIG. 1A, in another embodiment, the programinstructions are configured to cause the one or more processors toutilize focus and/or astigmatism sweeping across a single image indetermining the best focus of SEM sub-system 101. In one embodiment,focus and/or astigmatism may be varied during the acquisition of asingle image frame, and the point in the image frame that has best focusis used to determine the best focus setting.

In this regard, rather than induce a focus (or astigmatism) gradientspatially across an image field, the variation can be inducedtemporally. For instance, as one or more images are being acquired, themulti-lens assembly 109 may sweep the focus (e.g., swept fromunder-focused to over-focused) and/or astigmatism during acquisition ofone or more images. Then, the controller 110 may analyze at what pointin the one or more images the features are sharpest. Based on thisanalysis the best focus setting can be determined. It is noted that suchan approach is more likely to succeed in a multi-beam SEM because thelarger number of images will provide greater image information and areless likely to have gaps where no image information is available. FIG. 5depicts a conceptual view of a single image (of a set of images)acquired by detector assembly 108, which was acquired while sweeping thefocus from an under-focused condition to an over-focused condition. Forexample, image feature 502 a was acquired in an under-focused condition,while image feature 502 c was acquired in an over-focused condition.Image feature 502 b was acquired in an intermediate focus condition,which corresponds to the best available focus. It is noted herein thatthe accuracy in determining the best focus in this embodiment improveswith a higher number of resolvable structural features.

Referring again to FIG. 1A, in another embodiment, the programinstructions are configured to cause the one or more processors to tracka drift of a best focus condition. In one embodiment, the multi-lensassembly 109 and detector assembly 108 may periodically acquire imagesat a slightly under-focused condition, alternated with a slightlyover-focused condition. Then, the controller 110 may analyze the imagesto track a slow drift in the best focus condition.

In a review or inspection application where many images are acquired ona relatively flat sample, it may not be necessary to focus every image.In some embodiments, focus may be characterized over time as a slowdrift (e.g., drift due to charging, thermal drifts, sample tilt, and thelike) that only needs to be periodically corrected. In this case, theSEM sub-system 101 may periodically acquire a first image (or images)that is over-focused, alternated by periodically acquiring a secondimage (or images) that is under-focused. By comparing the under-focusedimage to the over-focused image, the controller 110 can determine if thefocus is drifting, and in which direction and by how much, and makeappropriate corrections. FIG. 6 illustrates a set of image features thatare acquired in an under-focused condition 600, a best focus condition602, and an over-focused condition 604. It is noted that by comparingthe current focus images 602 (presumed to be initially at a best focus)to periodically acquired under-focused images 600 and over-focusedimages 604 the controller 110 may determine if the current focus isdrifting and whether it is drifting to an under-focused condition or anover-focused condition. In turn, in the event the controller 110identifies focus drift, the controller 110 may direct the multi-lensassembly 109 to adjust the focus of one of more of the beams 105 tocompensate for the identified drift.

Referring again to FIG. 1A, in another embodiment, the programinstructions are configured to cause the one or more processors toaverage one or more images to identify best focus and/or settings tominimize astigmatism.

In one embodiment, the controller 110 may utilize information frommultiple sub-images or repeat sub-images images to allow focusing onimages that are otherwise too noisy for conventional focus algorithms.In one embodiment, the controller 110 may utilize information frommultiple sub-images or repeat sub-images to allow focusing on imagesthat have fewer pixels than would normally be required for goodfocusing.

It is noted that in the case of the multi-beam SEM sub-system 101 theset of focus images can be acquired at a much higher frame rate thanthat of a single beam SEM. In some cases, this may result in noisierimages that are more difficult to focus. However, by averaging focusresults across all sub-images (or a sub-set of sub-images) an accurateresult can still be obtained. In another embodiment, image frames may beacquired with few numbers of pixels (per sub-image). In this case, theuse of multiple channels of the multi-beam SEM sub-system 101 providesfor enough pixels and image data for accurate focus determination.

In one embodiment, the SEM sub-system 101 may acquire two or more“repeat” images for each channel defined by the primary beams 105 andsignal beams 117. Further, the controller 110 may combine (e.g.,average) the multiple repeat images for each channel of the SEMsub-system 101 to form an aggregated image for each of the multiplechannels of the SEM sub-system 101. For example, rather than take oneslow image that takes 1 second to acquire, 10 fast images, each 1/10 ofa second in duration, are acquired and averaged to create a singlehigh-quality image. Based on this averaged image a focus determinationmay be made.

While much of the present disclosure has focused on focus adjustments ofthe multi-beam SEM sub-system 101 using multiple images with themultiple beams 105, this should not be interpreted as a limitation onthe scope of the present disclosure. Rather, it is contemplated hereinthat the system 100 may be extended to achieve rapid focus adjustmentsin a multi-beam sub-system 101 in a variety of ways.

In one embodiment, the system 100 may carry out focus adjustments on themultiple beams 105 of the multi-beam SEM sub-system 101 using one ormore line scans. It is noted herein that the various embodiments andcomponents described previously herein with respect to image-basedfocusing may be extended to any of the embodiments of the presentdisclosure related to line scanning based focusing.

In one embodiment, the multi-beam SEM sub-system 101 may cause two ormore of the electron beams 105 to scan a selected pattern across thesample. Then, the detector assembly 108 may acquire line scaninformation from each of the corresponding signal beams 117. Based onthe signal beams 117 measured by the detector assembly 108, thecontroller 110 may determine the best focus condition (utilizing anymechanism described previously herein).

The one or more focus targets used to analyze the focus quality of theline scans may be found through any suitable process. In one embodiment,the one or more focus targets are located using information from a CADdatabase associated with the sample 107. In another embodiment, the oneor more focus targets are located using prior information associatedwith the stage location during recipe setup. In another embodiment, theone or more focus targets are located using prior associated receivedfrom an additional tool or system (e.g., additional inspection tool).

In another embodiment, the one or more focus targets may be identifiedwith no prior information. In one embodiment, the electron beams 105 maybe scanned in a selected pattern in an effort to locate one or morefocus target. For example, the electron beams 105 may be scanned in ageometric pattern, such as, but not limited to, a set of lines (e.g.,parallel lines, crossing lines and etc.). By way of another example, theelectron beams 105 may be scanned in a random pattern. It is noted thatsome topological features (used as focus targets) of the sample 107 willmost likely be identified by one or more beams 105 as the beams arescanned across the sample.

In one embodiment, the placement of the line scans may be selected so asto cross one or more features of the sample 107. In this regard, two ormore of the beams 105 may intersect at least one edge of the one or morefeatures. For example, the system 100 may acquire line scan informationfrom one or more pre-selected focus targets of the sample 107. Forinstance, the one or more pre-selected focus targets may be similarfeatures and/or have similar edge slopes. In this regard, two or morebeams 105 may scan lines across one or more pre-selected focus targetsor at least across one edge of one or more pre-selected focus targets.The detector assembly 108 may then receive the corresponding signalbeams and the controller 110 may measure edge information of thepre-selected focus targets for each of the two or more beams. In turn,based on the line scan information, the controller 110 may determine thebest focus condition.

It is noted that by analyzing the sharpness of multiple line scans asthe corresponding beams 105 are scanned across the edge of a feature (orsimilar features) the controller 110 can analyze relative focus of thebeams by analyzing the sharpness or abruptness of the line scan signalas it transitions across the feature edge. For instance, an out-of-focusbeam may result in a line scan that shows a blurred transition acrossthe edge of a feature, whereby an in-focus beam shows a clearly definedtransition across the edge of the feature. Further, the signal slope (asa function of position) of an in-focus beam will be larger than a signalslope for an out-of-focus beam when scanned across the same (or similarfeatures). As such, the controller 110 may identify the best focuscondition of the two or more line scans by analyzing the signal slopemeasured by the detector assembly 108 across one or more features of thesample 107.

FIG. 7 depicts a set of under-focused, in-focus and over-focused linescans across an edge 703 of a feature 702 of sample 107. Here, thefeature 702 serves as a focus target. As depicted, the under-focusedbeam 706 a and the over-focused beam 706 c will be less able to resolvethe edge 703 of the feature 702 than the in-focus beam 706 b. As aresult, the slope of the signal associated with in-focus beam 706 b willbe greater (i.e., more abrupt change from sample surface to top offeature) than that of the under-focused beam 706 a or the over-focusedbeam 706 c. Based on this analysis, the controller 110 may then identifythe beam having the greater slope being the beam with the best focus.

In another embodiment, two or more line scans of the same or similarfeature may be aligned and then averaged prior to analysis by thecontroller 110 in order to reduce noise. Such an alignment and averagingscheme may provide for faster focus data acquisition and increase theoverall scanning speed of the system 100.

In another embodiment, the focus of one or more electron beams used toperform a line scan may be swept. In this regard, the focus sweepingapproach described previously herein may be applied to a line scancontext. In another embodiment, the line scan focus sweeping approachdescribed above may be applied to cell-to-cell images using differentelectron beams 105.

Referring again to FIG. 1A, it is noted herein that the sample stage 106of the multi-beam SEM sub-system 101 may include any sample stage knownin the art suitable for securing the sample 107. The sample 107 mayinclude any sample suitable for inspection/review with electron-beammicroscopy, such as, but not limited to, a substrate. The substrate mayinclude, but is not limited to, a silicon wafer. In another embodiment,the sample stage 106 is an actuatable stage. For example, the samplestage 106 may include, but is not limited to, one or more translationalstages suitable for selectively translating the sample 107 along one ormore linear directions (e.g., x-direction, y-direction and/orz-direction). By way of another example, the sample stage 106 mayinclude, but is not limited to, one or more rotational stages suitablefor selectively rotating the sample 107 along a rotational direction. Byway of another example, the sample stage 106 may include, but is notlimited to, a rotational stage and a translational stage suitable forselectively translating the sample along a linear direction and/orrotating the sample 107 along a rotational direction. It is noted hereinthat the system 100 may operate in any scanning mode known in the art.

The detector assembly 108 of the multi-beam SEM sub-system 101 mayinclude any detector assembly known in the art suitable for detectingmultiple electron signals from the surface of the sample 107. In oneembodiment, the detector assembly 108 includes an electron detectorarray. In this regard, the detector assembly 108 may include an array ofelectron-detecting portions. Further, each electron-detecting portion ofthe detector array of the detector assembly 108 may be positioned so asto detect an electron signal from sample 107 associated with one of theincident electron beams 105. In this regard, each channel of thedetector assembly 108 corresponds to a particular electron beam of themultiple electron beams 105.

It is noted that the detector assembly 108 may be, but is not limitedto, a secondary electron detector or a backscattered electron detector.The detector assembly 108 may include any type of electron detectorknown in the art. For example, the detector assembly 108 may include amicro-channel plate (MCP), a PIN or p-n junction detector array, suchas, but not limited to, a diode array or avalanche photo diodes (APDs).By way of another example, the detector assembly 108 may include a highspeed scintillator/PMT detector.

The electron-optical assembly 104 may include any electron-opticalassembly known in the art suitable for illuminating a sample withmultiple electron beams and acquiring multiple images associated withthe multiple electron beams. In one embodiment, the electron-opticalassembly 104 includes a set of electron-optical elements for directingthe multiple electron beams 105 onto the surface of the sample 107. Theset of electron-optical elements may form an electron-optical column.The set of electron-optical elements of the electron-optical column maydirect at least a portion of the electron beams 105 onto multipleportions of the sample 107. The set of electron-optical elements mayinclude any electron-optical elements known in the art suitable forfocusing and/or directing the primary electron beams 105 onto thevarious areas of the sample 107. In one embodiment, the set ofelectron-optical elements includes one or more electron-optical lenses.For example, the one or more electron-optical lenses may include, butare not limited to, one or more condenser lenses (e.g., magneticcondenser lens) for collecting electrons from the multi-beam source 103.By way of another example, the one or more electron-optical lenses mayinclude, but are not limited to, one or more objective lenses 114 (e.g.,magnetic objective lens) for focusing the primary electron beams 105onto the various areas of the sample 107.

In another embodiment, the electron-optical assembly 104 includes a setof electron-optical elements for collecting electrons (e.g., secondaryelectrons and/or backscattered electrons) emanating from the sample 107in response to the multiple primary electron beams 105 and directingand/or focusing those electrons to the detector assembly 108. Forexample, the electron-optical assembly 104 may include, but is notlimited to, one or more projection lenses 115 for focusing the multipleelectron signal beams 117 to form multiple images of the variousportions of the sample 107 at the detector assembly 108.

It is noted that the electron-optical assembly 104 of system 100 is notlimited to the electron-optical elements depicted in FIG. 1A, which areprovided merely for illustrative purposes. It is further noted that thesystem 100 may include any number and type of electron-optical elementsnecessary to direct/focus the multiple beams 104 onto the sample 107and, in response, collect and image the corresponding signal beams 117onto the detector assembly 108.

For example, the electron-optical assembly 104 may include one or moreelectron beam scanning elements (not shown). For instance, the one ormore electron beam scanning elements may include, but are not limitedto, one or more electromagnetic scanning coils or electrostaticdeflectors suitable for controlling a position of the beams 105 relativeto the surface of the sample 107. Further, the one or more scanningelements may be utilized to scan the electron beams 105 across thesample 107 in a selected pattern.

By way of another example, the electron-optical assembly 104 may includea beam separator (not shown) to separate the multiple electron signalsemanating from the surface of the sample 107 from the multiple primaryelectron beams 105.

The one or more processors 116 of controller 110 may include anyprocessing element known in the art. In this sense, the one or moreprocessors 116 may include any microprocessor-type device configured toexecute software algorithms and/or instructions. In one embodiment, theone or more processors 116 may consist of a desktop computer, mainframecomputer system, workstation, image computer, parallel processor, or anyother computer system (e.g., networked computer) configured to execute aprogram configured to operate the system 100, as described throughoutthe present disclosure. It should be recognized that the steps describedthroughout the present disclosure may be carried out by a singlecomputer system or, alternatively, multiple computer systems. Ingeneral, the term “processor” may be broadly defined to encompass anydevice having one or more processing elements, which execute programinstructions from the non-transitory memory medium 118.

The memory medium 118 may include any storage medium known in the artsuitable for storing program instructions executable by the associatedone or more processors 116. For example, the memory medium 118 mayinclude a non-transitory memory medium. The memory medium 118 mayinclude, but is not limited to, a read-only memory, a random accessmemory, a magnetic or optical memory device (e.g., disk), a magnetictape, a solid state drive and the like. It is noted herein that thememory medium 118 may be configured to store one or more results fromthe detector assembly 108 and/or the output of one or more of thevarious steps described herein. It is further noted that memory medium118 may be housed in a common controller housing with the one or moreprocessors 116. In an alternative embodiment, the memory medium 118 maybe located remotely with respect to the physical location of the one ormore processors 116. For instance, the one or more processors 116 mayaccess a remote memory (e.g., server), accessible through a network(e.g., internet, intranet and the like).

The embodiments of the system 100 illustrated in FIG. 1A may be furtherconfigured as described herein. In addition, the system 100 may beconfigured to perform any other step(s) of any of the methodembodiment(s) described herein.

FIG. 8 is a flow diagram illustrating steps performed in a method 800 ofperforming a focus adjustment of a multi-beam SEM system, in accordancewith one or more embodiments of the present disclosure. It is notedherein that the steps of method 800 may be implemented all or in part bythe system 100. It is further recognized, however, that the method 800is not limited to the system 100 in that additional or alternativesystem-level embodiments may carry out all or part of the steps ofmethod 800. In step 802, a set of images of a surface of a sample aresimultaneously acquired with a multi-lens array. In step 804, a bestfocus (or best astigmatism) image of the set of images is identified. Instep 804, the multi-lens array is directed to adjust a focus condition(or astigmatism condition) of one or more electron beams based on thefocus settings of an electron beam corresponding with the identifiedbest focus (or best astigmatism) image.

FIG. 9 is a flow diagram illustrating steps performed in a method 900 ofperforming a focus adjustment of a multi-beam SEM system throughapplication of a focus gradient, in accordance with one or moreembodiments of the present disclosure. It is noted herein that the stepsof method 900 may be implemented all or in part by the system 100. It isfurther recognized, however, that the method 900 is not limited to thesystem 100 in that additional or alternative system-level embodimentsmay carry out all or part of the steps of method 900. In step 902, afocus gradient is established across an image field of a sample with aset of electron beams formed with a multi-lens array. In step 904, a setof images of a surface of a sample are simultaneously acquired with amulti-lens array. In step 906, a best focus (or best astigmatism) imageof the set of images is identified. In step 908, the multi-lens array isdirected to adjust a focus condition (or astigmatism condition) of oneor more electron beams based on the focus settings of an electron beamcorresponding with the identified best focus image.

FIG. 10 is a flow diagram illustrating steps performed in a method 1000of performing a focus adjustment of a multi-beam SEM system utilizingone or more focus sweeps, in accordance with one or more embodiments ofthe present disclosure. It is noted herein that the steps of method 1000may be implemented all or in part by the system 100. It is furtherrecognized, however, that the method 1000 is not limited to the system100 in that additional or alternative system-level embodiments may carryout all or part of the steps of method 1000. In step 1002, the focus ofone or more lenses is swept during acquisition of one or more images. Instep 1004, a point in the one or more images displaying a best focus isidentified. In step 1006, the multi-lens array is directed to adjust afocus condition of one or more electron beams based on the focussettings of an electron beam at the point in the one or more imagesdisplaying the identified best focus.

FIG. 11 is a flow diagram illustrating steps performed in a method 1100of measuring and compensating for focus drifting in a multi-beam SEMsystem, in accordance with one or more embodiments of the presentdisclosure. It is noted herein that the steps of method 1100 may beimplemented all or in part by the system 100. It is further recognized,however, that the method 1100 is not limited to the system 100 in thatadditional or alternative system-level embodiments may carry out all orpart of the steps of method 1100. In step 1102, a first image isacquired in an under-focused condition and an additional image isacquired in an over-focused condition. In step 1104, focus drift isidentified in a current image by comparing the first image and theadditional image to the current image acquired at the current focus. Instep 1106, a multi-lens array is directed to adjust a focus of one ormore electron beams to compensate for the identified focus drift.

FIG. 12 is a flow diagram illustrating steps performed in a method 1200of performing a focus adjustment in a multi-beam SEM system, inaccordance with one or more embodiments of the present disclosure. It isnoted herein that the steps of method 1200 may be implemented all or inpart by the system 100. It is further recognized, however, that themethod 1200 is not limited to the system 100 in that additional oralternative system-level embodiments may carry out all or part of thesteps of method 1200. In step 1202, a set of line scans are acquired. Instep 1204, a best focus (or best astigmatism) line scan is identified byanalyzing one or more line scan parameters (e.g., signal slope) of theline scans. In step 1206, the multi-lens array is directed to adjust afocus condition (or astigmatism condition) of one or more electron beamsbased on the focus settings of an electron beam corresponding with theidentified best focus (or best astigmatism) line scan.

All of the methods described herein may include storing results of oneor more steps of the method embodiments in the memory medium 118. Theresults may include any of the results described herein and may bestored in any manner known in the art. After the results have beenstored, the results can be accessed in the memory medium and used by anyof the method or system embodiments described herein, formatted fordisplay to a user, used by another software module, method, or system,etc. Furthermore, the results may be stored “permanently,”“semi-permanently,” temporarily, or for some period of time.

Those having skill in the art will recognize that the state of the arthas progressed to the point where there is little distinction leftbetween hardware and software implementations of aspects of systems; theuse of hardware or software is generally (but not always, in that incertain contexts the choice between hardware and software can becomesignificant) a design choice representing cost vs. efficiency tradeoffs.Those having skill in the art will appreciate that there are variousvehicles by which processes and/or systems and/or other technologiesdescribed herein can be effected (e.g., hardware, software, and/orfirmware), and that the preferred vehicle will vary with the context inwhich the processes and/or systems and/or other technologies aredeployed. For example, if an implementer determines that speed andaccuracy are paramount, the implementer may opt for a mainly hardwareand/or firmware vehicle; alternatively, if flexibility is paramount, theimplementer may opt for a mainly software implementation; or, yet againalternatively, the implementer may opt for some combination of hardware,software, and/or firmware. Hence, there are several possible vehicles bywhich the processes and/or devices and/or other technologies describedherein may be effected, none of which is inherently superior to theother in that any vehicle to be utilized is a choice dependent upon thecontext in which the vehicle will be deployed and the specific concerns(e.g., speed, flexibility, or predictability) of the implementer, any ofwhich may vary. Those skilled in the art will recognize that opticalaspects of implementations will typically employ optically-orientedhardware, software, and or firmware.

Those skilled in the art will recognize that it is common within the artto describe devices and/or processes in the fashion set forth herein,and thereafter use engineering practices to integrate such describeddevices and/or processes into data processing systems. That is, at leasta portion of the devices and/or processes described herein can beintegrated into a data processing system via a reasonable amount ofexperimentation. Those having skill in the art will recognize that atypical data processing system generally includes one or more of asystem unit housing, a video display device, a memory such as volatileand non-volatile memory, processors such as microprocessors and digitalsignal processors, computational entities such as operating systems,drivers, graphical user interfaces, and applications programs, one ormore interaction devices, such as a touch pad or screen, and/or controlsystems including feedback loops and control motors (e.g., feedback forsensing position and/or velocity; control motors for moving and/oradjusting components and/or quantities). A typical data processingsystem may be implemented utilizing any suitable commercially availablecomponents, such as those typically found in datacomputing/communication and/or network computing/communication systems.

It is believed that the present disclosure and many of its attendantadvantages will be understood by the foregoing description, and it willbe apparent that various changes may be made in the form, constructionand arrangement of the components without departing from the disclosedsubject matter or without sacrificing all of its material advantages.The form described is merely explanatory, and it is the intention of thefollowing claims to encompass and include such changes.

What is claimed:
 1. A multi-beam scanning electron microscopy apparatuscomprising: a multi-beam scanning electron microscopy sub-systemcomprising: a multi-beam electron source configured to form a pluralityof electron beams; a sample stage configured to secure a sample; anelectron-optical assembly including a set of electron-optical elementsconfigured to direct at least a portion of the plurality of electronbeams onto a portion of the sample; and a detector assembly configuredto simultaneously acquire a plurality of images of the surface of thesample, each image associated with an electron beam of the plurality ofelectron beams; a controller including one or more processors configuredto execute a set of program instructions stored in memory for causingthe one or more processors to: receive the plurality of images from thedetector assembly; disregard one or more images lacking sufficientstructural features to determine at least one of focus or astigmatism ofthe disregarded one or more images; identify at least one of a bestfocus image or a best astigmatism image of a plurality of remainingimages by analyzing one or more image quality parameters of at leastsome of the plurality of the remaining images; and direct the multi-beamsource to adjust at least one of focus or astigmatism of one or moreelectron beams based on at least one of focus or astigmatism of anelectron beam corresponding with at least one of the identified bestfocus image or the identified best astigmatism image.
 2. The apparatusof claim 1, wherein two or more of the images acquired by the detectorassembly have different foci.
 3. The apparatus of claim 2, wherein thedifferent foci are established unintentionally by one or more componentsof the multi-beam scanning electron microscopy sub-system.
 4. Theapparatus of claim 2, wherein the different foci are establishedintentionally by one or more components of the multi-beam scanningelectron microscopy sub-system.
 5. The apparatus of claim 1, wherein twoor more of the images acquired by the detector assembly have differentamounts of astigmatism.
 6. The apparatus of claim 5, wherein thedifferent amounts of astigmatism are established unintentionally by oneor more components of the multi-beam scanning electron microscopysub-system.
 7. The apparatus of claim 5, wherein the different amountsof astigmatism are established intentionally by one or more componentsof the multi-beam scanning electron microscopy sub-system.
 8. Theapparatus of claim 1, wherein the controller is configured to identify abest focus image of the plurality of images by analyzing image sharpnessof at least some of the images of the plurality of the images.
 9. Theapparatus of claim 1, wherein the controller is configured to: prior toidentifying best focus of the plurality of images, average two or moreimages.
 10. The apparatus of claim 9, wherein the controller isconfigured to: prior to identifying best focus of the plurality ofimages, average two or more repeat images.
 11. The apparatus of claim 1,wherein the controller is configured to: prior to identifying bestastigmatism of the plurality of images, average two or more images. 12.The apparatus of claim 11, wherein the controller is configured to:prior to identifying best astigmatism of the plurality of images,average two or more repeat images.
 13. The apparatus of claim 1, whereinthe multi-beam electron source comprises: an electron gun configured toemit an illumination beam; and a multi-lens array assembly configured tosplit the illumination beam into the plurality of electron beams. 14.The apparatus of claim 1, wherein the set of electron-optical elementscomprise: at least one of a condenser lens or objective lens.
 15. Theapparatus of claim 1, wherein the detector assembly comprises: adetector array.
 16. The apparatus of claim 1, wherein the detectorassembly comprises: one or more secondary electron detectors.
 17. Theapparatus of claim 1, wherein the detector assembly comprises: one ormore backscattered electron detectors.
 18. A multi-beam scanningelectron microscopy apparatus comprising: a multi-beam scanning electronmicroscopy sub-system comprising: a multi-beam electron beam sourceincluding an electron gun configured to generate an illumination beamand a multi-lens array assembly configured to split the illuminationbeam into a plurality of electron beams, wherein the multi-lens arrayassembly is configured for adjusting focus of one or more lenses of thelens array assembly; a sample stage configured to secure a sample; anelectron-optical assembly a set of electron-optical elements configuredto direct at least a portion of the plurality of electron beams onto aportion of the sample; and a detector assembly configured tosimultaneously acquire a plurality of images of the surface of thesample, each image associated with an electron beam of the plurality ofelectron beams; a controller including one or more processors configuredto execute a set of program instructions stored in memory for causingthe one or more processors to: direct the multi-beam scanning electronmicroscopy sub-system to simultaneously acquire a first plurality ofimages in an under-focused condition, a second plurality of images in anover-focused condition, and a current plurality of images at a currentfocus; receive the first plurality of images acquired in theunder-focused condition, the second plurality of images acquired in theover-focused condition, and the current plurality of images from thedetector assembly; identify focus drift in one or more of the currentplurality of images by comparing one or more of the first plurality ofimages and one or more of the second plurality of images to of the oneor more of the current plurality of images acquired at the currentfocus; and direct the multi-lens array to adjust a focus of one or moreelectron beams to compensate for the identified focus drift.
 19. Amulti-beam scanning electron microscopy apparatus comprising: amulti-beam scanning electron microscopy sub-system comprising: aplurality of multi-beam electron sources configured to form a pluralityof electron beams; a sample stage configured to secure a sample; anelectron-optical assembly including a set of electron-optical elementsconfigured to direct at least a portion of the plurality of electronbeams onto a portion of the sample; and a detector assembly configuredto simultaneously acquire a plurality of images of the surface of thesample, each image associated with an electron beam of the plurality ofelectron beams; a controller including one or more processors configuredto execute a set of program instructions stored in memory for causingthe one or more processors to: receive the plurality of images from thedetector assembly; disregard one or more images lacking sufficientstructural features to determine at least one of focus or astigmatism ofthe disregarded one or more images; identify at least one of a bestfocus image or a best astigmatism image of a plurality of remainingimages by analyzing one or more image quality parameters of at leastsome of the plurality of remaining images; and direct one or moreelectron-optical elements to adjust at least one of focus or astigmatismof one or more electron beams based on at least one of focus orastigmatism of an electron beam corresponding with at least one of theidentified best focus image or the identified best astigmatism image.20. A multi-beam scanning electron apparatus comprising: a multi-beammicroscopy sub-system to perform a line scan of a plurality of electronbeams comprising: a multi-beam electron source configured to form theplurality of electron beams; a sample stage configured to secure asample; an electron-optical assembly including a set of electron-opticalelements configured to direct at least a portion of the plurality ofelectron beams onto a portion of the sample; and a detector assemblyconfigured to simultaneously acquire a plurality of images of thesurface of the sample, each image associated with an electron beam ofthe plurality of electron beams; a controller including one or moreprocessors configured to execute a set of program instructions stored inmemory for causing the one or more processors to: receive a plurality ofline scans from the detector assembly; disregard one or more line scanslacking sufficient structural features to determine at least one offocus line scan or astigmatism line scan of the disregarded one or moreline scans; identify at least one of a best focus line scan or a bestastigmatism line scan of a plurality of remaining line scans byanalyzing one or more line scan parameters of at least some of theplurality of remaining line scans; and direct the multi-lens source toadjust at least one of focus or astigmatism of one or more electronbeams based on at least one of focus or astigmatism of an electron beamcorresponding with at least one of the identified best focus line scanor the identified best astigmatism line scan.